Conventionally, in the field of transmission device for work vehicles, power from an engine was distributed into two courses; in one course power was transmitted to a planetary gear mechanism; in the other course power was transmitted to the planetary gear mechanism after transmitted to a Hydraulic Static Transmission (here in after referred to as “HST”), steplessly converting the speed thereof; and then, power from those two courses were combined by the planetary gear mechanism and outputted. Comprising the transmission device for work vehicles as described above has become public knowledge (for example, see the Patent Literature 1).
In order to make the transmission device capable of being used with high-horse power engines, it is necessary to (A) mount large volumetric HST on the work vehicles, or (B) use more than two planetary gear mechanisms to compose a complex power transmission mechanism.
However, using a means (A) or (B) as described above or the like have recognized disadvantageous in that it accompanies a loss of power transmission efficiency or fuel economy. Moreover, using those means have recognized disadvantageous in that the size of the transmission device becomes larger and it will be difficult to mount such transmission device on work vehicles.